Other source:
As the battle between Trunks and Cell rages on, Cell reveals Trunks's weakness: in his ascended form, his new bulk slows him down. Goku, having ascended in the Hyperbolic Time Chamber, also realizes this. He plans to spend as much time as possible as a Super Saiyan to eliminate this weakness. Vegeta recovers, and returns to the island to resume his battle with Cell. Cell, having the same desire for an interesting fight as most of the Z warriors and their allies, cannot resist the idea of facing the Z fighters at their best, so he decides to revive the World Martial Arts Tournament for just this purpose.
